The present invention relates to surfactant, particularly viscoelastic surfactant based gelling compositions preferably used for wellbore service fluids. More particularly it relates to such compositions for selectively reducing the flow of subterranean aqueous fluids into a well while maintaining the hydrocarbon production.
Various types of wellbore fluids are used in operations related to the development, completion, and production of natural hydrocarbon reservoirs. The operations include fracturing subterranean formations, modifying the permeability of subterranean formations, or sand control. Other applications comprise the placement of a chemical plug to isolate zones or complement an isolating operation. The fluids employed by those operations are known as drilling fluids, completion fluids, work over fluids, packer fluids, fracturing fluids, conformance or permeability control fluids and the like.
Of particular interest with regard to the present inventions are fluids for water control applications: During the life cycle of a hydrocarbon well, e.g., a well for extracting oil or natural gas from the Earth, the producing well commonly also yields water. In these instances, the amount of water produced from the well tends to increase over time with a concomitant reduction of hydrocarbon production. Frequently, the production of water becomes so profuse that remedial measures have to be taken to decrease the water/hydrocarbon production ratio. As a final consequence of the increasing water production, the well has to be abandoned.
In many cases, a principal component of wellbore service fluids are gelling compositions, usually based on polymers or viscoelastic surfactants.
There has been considerable interest in the viscoelastic gels formed from the solutions of certain surfactants when the concentration significantly exceeds the critical micelle concentration. Viscoelastic surfactant solutions are usually formed by the addition of certain reagents to concentrated solutions of surfactants, which most frequently consist of long-chain quaternary ammonium salts such as cetyltrimethylammonium bromide (CTAB). Common reagents which generate viscoelasticity in the surfactant solutions are salts such sodium salicylate and sodium isocyanate and non-ionic organic molecules such as chloroform. The electrolyte content of surfactant solutions is also an important control on their viscoelastic behaviour. The viscoelastic properties of a solution arises from the formation of long cylindrical (or xe2x80x9cworm-likexe2x80x9d) micelles and their entanglement to form a three-dimensional structure. The surfactant micelles behave in a manner somewhat similar to polymer chains, although the former are dynamic entities with the surfactant monomers constantly joining and leaving the micelles. The micelles are held together by van der Waals (and other similar) interactions, in contrast to the strong covalent bonds between monomer units in polymers. The surfactant micelles are fragile (4) and the gels formed by the entangled micelles are relatively weak. Such gels are often termed physical gels (6), in contrast to chemical gels which are commonly formed by the cross-linking of high molecular weight polymers using covalent or ionic bonds.
Further references related to the use of viscoelastic surfactants as wellbore service fluids can be found for example in U.S. Pat. No. 4,695,389, U.S. Pat. No. 4,725,372, U.S. Pat. No. 5,258,137 and U.S. Pat. No. 5,551,516.
Several patents have described the use of polymerizable surfactants in emulsion polymerizatio. In U.S. Pat. No. 5,162,475, there is described the use of xcex1-xcex2 (i.e., terminal) ethylenically unsaturated poly(alkyleneoxy) compounds which act as the surfactant in the emulsion polymerization process and which co polymerize with the non-surfactant monomers. Several earlier patents, U.S. Pat. No. 4,049,608, 4,224,455 and 4,337,185 have also described the co-polymerization of the surfactant monomers used in emulsion polymerization processes. In U.S. Pat. No. 4,064,091 there is described the use of unsaturated quaternary ammonium salts as surfactants in emulsion polymerization processes which co-polymerize with the non-surfactant monomers to produce self-stabilising polymeric dispersions which are free of surfactant monomers.
Most recent work by K. Tauer, published in xe2x80x9cPolymeric Dispersions: Principles and Applicationsxe2x80x9d (J. M. Asua ed.), NATO ASI Series E: Applied Sciences Vol. 335, 1997 describes the polymerization of surfactant-like monomers in small micelles in the absence of other surfactants.
The object of this present invention is to provide improved compositions, especially or wellbore service fluids, based on monomeric surfactants, particularly monomeric viscoelastic surfactants. It is a specific object of the invention to provide stable gels using such compositions. It is a further specific object of the invention to provide such compositions for water control operations in hydrocarbon wells.
The objects of the invention are achieved by polymerizing monomeric surfactants forming micelles in an aqueous solution.
Herein, monomeric is defined as having no repetitive units. Preferably, the molecular mass of monomeric surfactants is less than 10000, preferably less than 1000 units.
Surfactants are water soluble surface-active materials with a hydrophobic group. The solubility in water is controlled by a hydrophilic group. Surfactants are usually classified according to their electrochemical properties as anionic, cationic or non-ionic agents. Often they are referred to as detergents, soaps or amphiphilic compounds.
In solution (and above a critical concentration) surfactants form micelles. The concentration of the surfactants is sufficient to transform the solution into a gel. Preferably, the concentration lies in the range of 1 to 10 weight per cent.
In a preferred embodiment of the invention the momomeric surfactant belong to the class of surfactant which display in solution viscoelastic behavior.
The polymerizing agent is capable of initiating a polymerization of the surfactants forming a micelle, thus stabilizing the gel.
The concentration of the agent is preferably in the range of 10 to 1000 ppm (parts per million)
As applied to solutions, the term xe2x80x9cviscoelasticxe2x80x9d means a viscous solution which at least partially returns to its original state when an applied stress is released. The property of viscoelasticity can be tested for example by observing whether bubbles created by swirling the sample recoil after the swirling ceased. For this and other test reference is made to H. A. Barnes et al. Rheol. Acta. 14 (1975), pp. 53-60 and S. Gravsholt, Journal of Coll. and Interface Sci. 57(3), 1976, pp.575-6.
The physical gels formed by viscoelastic surfactant solutions can exhibit considerable responsiveness to their external chemical and physical environments. For example, the viscoelasticity of these concentrated surfactant solutions can be destroyed by contact with hydrocarbons and other organic liquids. The viscoelasticity of the solutions can also be lost on heating but recovered on cooling. However, once polymerized, the polymeric gels formed show significantly less responsiveness to their chemical and physical environment. Viscoelastic surfactants employed by the current invention are described for example in the above cited U.S. Pat. No. 4,695,389, U.S. Pat. No. 4,725,372, and U.S. Pat. No. 5,551,516 and literature referred to therein.
In a further preferred embodiment of the invention, the polymerized surfactants are cross-linked, thus further enhancing the stability of the gel. The preferred concentration of the cross-linking agent in the solution lies in the range of 10 to 1000 ppm.
Chemical cross-linking is defined as forming a chemical bond between the cross-linked molecules. Chemical cross-linking is understood to be stable and irreversible.
The cross-linking agents can be either inorganic ions (or ionic complexes) or polar organic molecules. When the polymer contains ionic groups such as carboxylate or sulphonate functions the polymer chains can be cross-linked by inorganic ions such as chromium(III) or zirconium(IV), frequently in the presence of monomeric ligands, such as acetate or adipate ions, to control the rate of cross-linking. Alternatively, organic cross-linking agents, such as hexanal or heptanal, can be used.
These and other features of the invention, preferred embodiments and variants thereof, and further advantages of the invention will become appreciated and understood by those skilled in the art from the detailed description following below.
The polymerization and cross-linking of the cylindrical micelles in viscoelastic surfactant solutions is illustrated with three examples. The polymerization in the three examples is achieved by a free radical mechanism. However, it is stressed that the polymerization of the surfactant monomers to form polymeric surfactants can be achieved by a number of well known methods, including ring-opening polymerization, cation polymerization and anionic polymerization techniques. A description of these and other polymerization techniques has been given by G. Odian in: xe2x80x9cPrinciples of Polymerizationxe2x80x9d, 3rd ed., pp., Wiley, N.Y. (1991).